Disposable pump cassettes are frequently employed to infuse medicinal fluids into a patient. As described in U.S. Pat. Nos. 4,818,186 and 5,000,664, one type of disposable cassette includes a plastic housing having a front and a rear portion, between which an integral elastic member is encapsulated. The housing has a plurality of ports through which rod-like actuators of a pump drive mechanism interact with the elastomeric membrane to control fluid flow through the cassette housing. A pump plunger on the drive unit displaces the membrane to pressurize liquid trapped in a pumping chamber formed between the membrane and the back of the housing. Similarly, rod-like actuators extend from the drive unit through ports in the housing, pressing against the membrane to interrupt fluid flow through valve passages formed in the back of the housing. A microprocessor in the pump drive controls the pump plunger and rod-like actuators to effect a desired rate of delivery of medicinal fluids to the patient, and in some units, is capable of selecting between a plurality of different sources by opening an appropriate selector valve in the cassette.
Selection of the source fluid and pumping rate or volume are normally determined by an operator programming the pump drive in response to a display prompt. Significant leakage through the valves in the pump cassette can create a potentially harmful variation from the programmed value in the quantity of medication actually delivered to a patient, or in the case of a leaky selector valve, may allow a medicinal fluid to enter the pump cassette when infusion of the fluid into the patient is not desired. Leakage of the valves is difficult or impossible to detect by visual inspection, and may occur after the cassette was originally inspected for leaks during its manufacture. In view of the potential harm to the patient should significant leakage go undetected, there is a clear justification for evaluating the leakage integrity of all valves when cassette is first used to administer drugs, and perhaps at periodic intervals thereafter, e.g., each time that the pump is energized.
Apparatus and a method for detecting valve leakage in a pump cassette are disclosed in U.S. Pat. No. 5,000,664. The cassette disclosed in this reference includes a primary valve and a secondary valve that are selectively activated to control the source of fluid input to the cassette. The cassette also includes an inlet valve and an outlet valve disposed on each side of a pumping chamber. Downstream of the outlet valve is disposed a pressure sensor, which produces a signal indicative of the pressure of fluid within the cassette at that point. Leakage in the inlet or outlet valves is detected by closing both valves, pressurizing fluid in the pumping chamber for a predetermined period of time, and then opening the outlet valve. If a pressure pulse having an amplitude less than a predetermined level is detected downstream of the outlet valve when it is opened, either the inlet or outlet valve has leaked. The primary and secondary valves are checked for leakage by pressurizing fluid trapped in the cassette, delaying for a period of time before closing the inlet valve, and then, opening the outlet valve to detect a pressure pulse propagating downstream of the outlet valve, using the pressure sensor. If a pressure pulse of less than the predetermined magnitude is detected, one of the primary or secondary fluid selectors is leaking. Both tests also detect leakage of other portions of the pump assembly.
The above-described apparatus and algorithm accurately detect valve leakage under most conditions. However, under certain conditions that were previously unrecognized, the above method can fail to detect valve leakage. When the valve leak test is performed on a cassette having valves that do not leak or on a cassette having a defective outlet valve, the results are as expected. However, when the valve leak test is performed on a cassette having a defective inlet valve, at times, a pressure pulse having a magnitude substantially greater than the predetermined minimum magnitude is unexpectedly observed. This false pulse can occur when the inlet valve is leaking because of the pressure head developed by the column of liquid between the pump cassette and the reservoir; the pressure of this head of liquid propagates to the pressure sensor when the outlet valve opens. Thus, for this condition, the defective inlet valve is not detected and leakage is allowed to continue to the detriment of the patient. The present invention ensures that a defective inlet valve is detected.